1. Field of the Invention
This invention relates to a positioning control system for accurate positioning of numerical-controlled machines.
2. Description of the Prior Art
A positioning control system heretofore employed for numercial-controlled machines (for example, as shown in FIG. 1) which employs a closed loop type servomechanism. With this conventional system, as shown in FIG. 1 velocity information detected by a tachogenerator or a like velocity detector T1, and positional information detected by a resolver or a like position detector R1, both detectors being mounted on a motor M, are fed back to the input side, wherein they are applied to adders AD1 and AD2, respectively. In the adder AD1, a comparison is made between commanded positional information from a control unit CONT and the detected positional information from the position detector R1, and the resulting difference signal is provided as a velocity command to the adder AD2 via an amplifier AMP1. In the adder AD2, the velocity command and the detected velocity information from the velocity detector T1 are compared, and the motor M is driven via a velocity amplifier AMP2 in such a manner as to reduce the difference between these quantities to zero, by which a feed screw V is driven to position a movable machine part MC.
In the conventional system described above, in which the velocity feedback is derived from the motor M, it is impossible to correct vibration of the mechanical system which is produced by its various elements at the end of the positioning operation; therefore, accurate positioning may in some cases be impossible. With the use of an arrangement in which the velocity feedback is derived directly from the mechanical system, the vibration can be suppressed. In general, however, when the mechanical system is included in the velocity feedback loop, various characteristics of the mechanical system exert an adverse influence on the velocity control loop; namely, even in a steady state in which no velocity difference exists between the motor M and the movable machine part MC, that is, when no vibration occurs, the characteristics of the mechanical system are fed back to make the velocity control loop unstable.
In the past, an attempt has been made to prevent vibration of the mechanical system by employment of a positioning control system such as depicted in FIG. 2. This system includes another feedback loop, for the positional information of the mechanical system, in the arrangement of FIG. 1. With this system, the position of the machine is detected by a position detector R2 mounted on the movable machine part MC, the commanded positional information is applied to a deviation set circuit SM to detect the expected rotational angle of the output shaft of the motor M, and then the two items of information are compared by adder AD3 to thereby detect an error of the whole mechanical system. The error signal thus detected is applied to a pulse oscillator OCL to yield correction pulses corresponding to the error. The correction pulses are provided to an adder AD4, wherein they are added to the commanded positional information from the control unit CONT, thereby correcting the commanded rotational angle of the motor M to eliminate the error. Since the error signal derived from the adder AD3 also includes an error resulting from vibration of the mechanical system, it is possible theoretically to suppress the vibration of the mechanical system; nevertheless, this system is intended, in general, to correct position errors, such as backlash error, pitch error and so forth. The gain of this system having the position detector R2 is limited by the natural frequency of the mechanical system, so that in the case of the natural frequency being low, the gain for a very small displacement is reduced; thus the response to a very small displacement (such as mechanical vibration or the like) is essentially retarded, resulting in a failure to sufficiently suppress the vibration. Moreover, the use of the deviation set circuit is disadvantageous economically.